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Animal Reproduction Science 60–61 2000 233–244 www.elsevier.comrlocateranireprosci
Twin reduction in the mare: current options
Margo L. Macpherson
a,
, Johanna M. Reimer
b
a
Department of Large Animal Clinical Sciences, College of Veterinary Medicine, UniÕersity of Florida, PO Box 100136, GainesÕille, FL 32610-0136, USA
b
Rood and Riddle Equine Hospital, PO Box 12070, Lexington, KY 40580, USA
Abstract
Twin pregnancy in the mare causes economic loss. Early transrectal ultrasonographic detection of twins and manual crush of one embryonic vesicle is the method of choice for managing equine
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twins 90 success rate . Transcutaneous and transvaginal ultrasound-guided twin reduction techniques are described for twin pregnancies that advance beyond 25 days of gestation. Reported
success rates for the ultrasound-guided procedures are 50 and 20, respectively. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Twins; Mare; Reduction; Transvaginal; Transcutaneous
1. Introduction
In spite of improved management techniques, twin pregnancy continues to be a source of economic loss in broodmare production. Twinning historically has accounted
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for 10–30 of abortions in the mare Roberts, 1986 . Recently, Giles et al., 1993 examined causes of abortion, stillbirth and perinatal death in horses between 1986 and
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1991. Of 3527 necropsy cases reported, 221 6 mares had aborted twins. Early diagnosis and management of twins has reduced the incidence of abortion in mares due
to twins; however, twin pregnancies still account for a notable amount of pregnancy wastage in the mare.
2. Management of equine twin pregnancies
Ultrasonographic examination of the reproductive tract early in gestation allows for Ž
. prompt diagnosis and treatment of twin pregnancies Chevalier and Palmer, 1982 .
Corresponding author. Tel.: q1-352-392-4700 ext. 4000; fax: q1-352-392-8289. Ž
. E-mail address: macphersonmmail.vetmed.ufl.edu M.L. Macpherson .
0378-4320r00r - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž
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Pregnancies are detected as early as Day 9 of gestation with the aid of transrectal Ž
. ultrasonography Ginther, 1986 . Diagnosis of twin pregnancies is optimally achieved
between Days 13 and 15 of gestation to ensure detection of twins arising from Ž
. Ž .
asynchronous ovulations ovulations occurring more than 24 h apart Ginther, 1989a .
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Twin pregnancies detected during the early mobility phase prior to Day 16 of gestation are best managed by crushing one embryonic vesicle, with a survival rate for the
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remaining twin exceeding 90 Roberts, 1982; Pascoe, 1983; Pascoe et al., 1987 . Ž
. During the period of embryonic fixation Days 16–17 of gestation , approximately
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70 of twin vesicles will fix in the same horn unilateral . Of unilaterally fixed twin pregnancies, approximately 85 culminate in natural reduction of one embryo prior to
Day 40 of gestation. The remaining embryo develops normally following natural Ž
. reduction. Ginther, 1984, 1989b .
Unilateral twins that do not undergo natural reduction to a singleton are challenging to manage. Manual reduction of a unilaterally fixed twin is difficult without damaging
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the remaining conceptus. Bowman 1987 and McKinnon and Rantanen 1998 reported Ž
. good success 90–95 with manual crush of a unilateral twin from Day 17 to 20 if the
vesicles could be separated. The technique was less advantageous for inseparable vesicles or unilateral twins G 20 days gestation.
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Bilaterally fixed twin pregnancies one conceptus in each uterine horn typically do Ž
not result in natural reduction to a singleton conceptus during early pregnancy Ginther, .
1984 . Instead, both fetuses often survive until later stages of gestation, at which time Ž
. Ž
. abortion usually occurs Jeffcott and Whitwell, 1973 . Ginther and Griffen 1994
recently followed bilateral twin pregnancies in 15 pony mares. In two mares, death of Ž
. one fetus occurred in the first two months of gestation. Eight of fifteen mares 67
aborted both fetuses by three months of gestation. Four mares aborted one or both fetuses at G 8 months and one mare delivered live twins. The authors noted that early
fetal death appeared to be temporally related to the apposition of allantochorions between the fetuses. The actual mechanism causing fetal death is currently unknown.
Manual crush of a bilateral twin results in reduction to a singleton pregnancy in 75 Ž
. of cases if performed before 30 days of gestation Pascoe et al., 1987 . Delayed attempts
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to reduce twin pregnancies i.e., beyond 30 days of gestation more likely results in Ž
abortion of both conceptuses with manual reduction techniques Bowman, 1987; Pascoe .
et al., 1987 . Management of equine twin pregnancies after 30 days gestation is further compli-
cated by the formation of endometrial cups at approximately Day 36 of gestation. The Ž
. Ž endometrial cup cells secrete the hormone equine chorionic gonadotropin eCG
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and Moor, 1972 . This hormone is reported to stimulate the formation of secondary corpora lutea thereby leading to increased progesterone production for pregnancy
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maintenance Cole et al., 1946; Amoroso et al., 1948 . Endometrial cups remain
functional until approximately Day 80–120 of gestation in either the presence or Ž
. Ž
. absence of a viable fetus Clegg et al., 1954; Mitchell, 1971 . Thompson et al. 1982
demonstrated that irregular estrous cycles and impaired fertility occurred with pregnancy loss in the presence of endometrial cups. Consequently, if twin pregnancies are not
successfully managed prior to cup formation and both pregnancies are lost after Day 35, the mare may not return to fertile estrus for a prolonged time. Methods for managing
equine twins beyond Day 30 of gestation have resulted in inconsistent outcomes. Ž
. Methods include dietary energy restriction Merkt et al., 1982 , needle aspiration of one
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Ž embryonic vesicle per vagina Pascoe, 1979 , surgical removal of one vesicle Pascoe
. and Stover, 1989 and transvaginal and transcutaneous ultrasound-guided fetal reduction
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techniques Rantanen and Kincaid, 1988; Bracher et al., 1993; Jonker et al., 1995 . Currently, ultrasound-guided techniques are commonly used methods for reduction of
fetal twins.
3. Transvaginal ultrasound-guided pregnancy reduction in mares
